Home theater systems, which once were expensive luxury items, are now becoming commonplace entertainment devices. Complete Home Theater systems, known as a Home Theater In a Box (HTIB), are available to consumers at reasonable prices. However, properly setting up such Home Theater systems can sometimes be problematic for the consumer.
Home theater systems provide a number of components, which may be located in various parts of the room. The components include the home theater receiver/amplifier, front stereo speakers (left and right), rear surround sound speakers (left and right), a center speaker, and a subwoofer. Various other combinations of speakers may also be used, including additional or fewer speakers. One such home theater system is described, for example, in U.S. Pat. No. 5,930,370, issued Jul. 27, 1999 to Ruzicka, incorporated herein by reference.
FIG. 1 depicts a diagrammatic view of the home theater surround sound speaker system (the surround sound system) 10 arranged in accordance with the principles of the present invention. The surround sound system 10 includes a source of a preferably amplified stereo signal, shown in FIG. 4 as television (“TV”) set 12. The stereo audio source may be any of a number of audio signal sources. It should, thus, be noted that the source of a stereo audio signal is represented herein as television 12, but the audio signal source may also be a stereo receiver, a car stereo, a portable compact disk or tape player, a portable boom-box type stereo, or any other source of a stereo signal.
Television 12 outputs an amplified audio signal to interconnect module 14 via a multi-conductor cable 16. Multi-conductor cable 16 typically includes two conductor pairs for conducting the left and right channels of the stereo signal output by television 12 to interconnect module 14. Interconnect module 14 receives the audio signals from television 12 and assembles the component left and right channel signals for selective distribution to particular component speakers of the surround sound system 10.
The component speakers typically include a sub-woofer 18, which receives full range left and right signals, but only reproduces the low frequency components of the audio signal. Interconnect module 14 also outputs an audio signal to front center speaker 20. Front center speaker 20 receives both the left and right component signals of the stereophonic signal and reproduces the (L+R) summation signal. Preferably, front center speaker 20 is located in proximity to television 12 and projects the acoustic output of the (L+R) summation signal toward the listener 28.
Interconnect module 14 also outputs the left channel signal to left satellite speaker 22 and right channel signal to right satellite speaker 24. Left satellite speaker 22 and right satellite speaker 24 may be relatively small speakers and need only reproduce mid range and/or high frequency signals. Left and right satellite speakers are preferably oriented so that the primary axis of radiation of the speaker points upward along a vertical axis; however, other orientations of the satellite speakers may also provide satisfactory performance. Interconnect module 14 also outputs an audio signal to rear ambience speaker 26. Rear ambience speaker 26 typically receives an audio signal in the form of a left channel minus right channel (L−R) or a right channel minus left channel (R−L) difference signal. As will become apparent throughout this detailed description, several embodiments of the invention described herein enable interconnect module 14 to generate a variety of signals to be output to left satellite speaker 22, right satellite speaker 24, and/or rear ambience speaker 26. It should be noted at the outset that the term speaker refers to a system for converting electrical input signals to acoustic output signals where the system may include one or a number of crossover networks and/or transducers.
The components described in FIG. 1 typically are arranged to optimize the surround sound effect to enhance the listening experience of the viewer 28. The viewer 28 typically faces television 12 which has front center speaker 20 arranged in proximity to television 12 so that center speaker 20 and television 12 radiate their respective audio and video output in the general direction of viewer 28. The left satellite speaker 22 typically is arranged to the left side of viewer 28 while right satellite speaker 24 is arranged to the right side of viewer 28, both satellite speakers typically being located nominally midway between the viewer 28 and television 12. Rear ambience speaker 26, which contributes to creating a spacious audio effect, is typically located behind viewer 28. Rear ambience speaker 26 is depicted as a single speaker, but multiple rear speakers 26 may be included in the system.
One problem with these home theater systems is in adjusting the equalization of the system to compensate for room acoustics, speaker type, and other factors. Traditionally, a consumer adjusted equalization using a so-called graphic equalizer, where a number of narrow band-pass filters are provided, each with a corresponding slide switch. The consumer adjusts each slide switch to attenuate or amplify a particular frequency band. More modern systems may use electronic displays in place of the slide switches, but the overall functionality is the same.
The purpose of an equalizer is to provide an audio response that is generally “flat” across the entire frequency spectrum. Due to limitations in system and speaker design, as well as room acoustics and interaction of room acoustics with speaker design and placement, various frequency ranges in a system may be attenuated or accentuated, resulting in a sound reproduction which is not faithful to the original recording.
A “flat” response generally refers to the resulting frequency versus amplitude graph. If the system is properly equalized, the graph should look like a flat line though all frequency ranges. In reality, this goal is never entirely achieved due to limitations of audio components and room acoustics. However, with a graphic equalizer, it is possible to improve the response considerably such that the resulting sound is a more faithful reproduction of the original sound.
The problem with manually operated graphic equalizers is that the equalization is based upon the consumer setting the various frequency levels based upon what the consumer hears and what the consumer thinks will create the proper equalization for the system. This manual solution is a largely empirical approach, as many consumers cannot properly isolate various frequencies “by ear” and understand how to adjust the equalizer properly. In addition, as the music type and sound changes, as well as the acoustic properties of the room, the equalization set at one level may be inappropriate for another audio environment.
The early graphic equalizers generally had a fixed number of equalizer circuits, each adjusting a predetermined narrow band of frequencies. However, when attempting to equalize a system, it becomes readily apparent that certain frequency ranges may require finer incremental ranges of adjustment, whereas whole bands of frequencies can be adequately adjusted using a single circuit. Proving additional equalizer filter elements and switches to solve this problem is prohibitively expensive. The parametric equalizer helps solve this problem by allowing a limited number of equalizer elements to adjust audio levels in a flexible manner. Each level adjustment may be itself adjusted to control a different frequency range.
Thus, each band-pass filter in the equalizer may be adjusted for width. Frequency ranges that require a fine granularity of adjustment may be more precisely controlled using a number of narrow-band elements in the parametric equalizer. Large frequency ranges that can be adjusted as a group can be controlled with one single wide-band element in the parametric equalizer. In this manner, the parametric equalizer can provide a more sophisticated and correct equalization to the frequency spectrum with the same number or even fewer control elements than a typical prior art fixed-frequency element graphic equalizer. Again, however, if a consumer attempts to manually control equalization, the results are often less than optimal, as the results are based upon the ability of the consumer to discern different frequency ranges.
Equalization can be achieved in software as well as in hardware. For example, when decoding a digital data stream, such as from a CD, DVD, or other digital audio source, equalization may be applied to the data as part of the decoding process or in a separate step. Thus, the process of equalization, either using a fixed-bandwidth graphic equalizer or a variable-bandwidth parametric equalizer, can be achieved in software as well as in hardware, or in a combination of both.
Other systems are known in the art wherein home theater systems, particularly more low-end units, provide a limited number of pre-set equalization patterns for different music types and listening styles. Thus, a home theater system may provide pre-set equalization levels for rock music, jazz, classical, rap, or for movie or DVD playback or the like. The equalization takes place in software within the home theater system. These pre-set levels do not take into account the room acoustics and provide only limited choices to the consumer. The consumer can only select the equalization setup that sounds best for the given circumstances. The system is not optimized for the room acoustics, speakers, and other factors affecting audio playback.
More recently, one of the more popular features for home theater systems has been some form of automatic equalization setup to minimize adverse affects of speaker/room interactions. Most solutions, however, involve a one-time setup performed by the user when installing the system and/or prior to listening to music and/or watching a video or the like. An example of such a prior art equalization system is illustrated in U.S. Pat. No. 6,721,428, issued Apr. 13, 2004 to Allred et al. and incorporated herein by reference.
These prior art automatic equalization setup systems typically have three phases. First, the system is analyzed from a single position or multiple positions in the room, usually by generating an audio signal through the speakers, and then receiving the audio signal through a remote microphone or the like to produce a system response. Second, the results of such analysis are translated to a run-time equalization setup and saved. Run-time equalization refers to the process of equalizing the audio signal during the digital decoding stage. Third, the saved settings are used by the equalizer at run-time as an additional post-processing step to whatever other audio processor software is running at the time.
Examples of such other audio processor software include Dolby™ Digital™ AC-3, Digital Theater Systems (DTS), Pulse Code Modulation (PCM), bass management, delay control or the like. These various digital audio processing algorithms are known in the art and may be licensed from their respective producers, or comparable algorithms may be devised. The equalization algorithms of the Prior Art may thus be applied subsequent to the decoding step in the playback of a digital audio stream from a DVD, CD, or other audio source. Such audio processor software may include an existing equalization algorithm, which may receive an input based upon system response in the room.
The extent to which the system can be corrected for the speaker/room acoustics is largely determined by the complexity of the run-time equalization. For many low- to mid-level systems, the run-time equalization is simply the parametric or graphic equalizer already present in the software, and thus correction possibilities may be limited. Thus, it remains a difficulty in the prior art as to how to best fit a fixed-band graphic equalizer or parametric equalizer to a predetermined frequency response.
There are a number of prior solutions to the problem of fitting a fixed-band equalizer to a predetermined frequency response. One solution is a straightforward curve-fitting. For an n-band equalizer, the algorithm finds the n highest peaks and/or valleys in the frequency response and sets each band to correct the corresponding feature. Thus, if a particular frequency range is too high, it may be attenuated, and if a particular frequency range is too low, it may be boosted.
There are at least two problems with the curve-fitting technique. Quite often the peaks found in a system response (here, the term “system response” refers to the response of the speaker and room) are at least partially due to phase-response issues, which may not respond in the desired manner to a frequency-based solution. Applying a −3 dB equalization to a 3 dB peak may not flatten the response as intended.
The “phase response” of the speaker in the room is a function of frequency and is one part of the frequency response. The other part is the magnitude response (often inaccurately called the “frequency response”), which is the power level (Y-axis, usually in dB) plotted against frequency (X-axis in Hz).
A second problem with the curve fitting technique is the limited granularity of the underlying equalizer. For run-time equalizers with a limited range of center frequencies (either a graphic equalizer or a limited-implementation of a parametric equalizer), it may not be possible to exactly “center” on the peak or valley in system response. Thus, more or less of the frequency response is affected as desired. If a −3 dB attenuation is applied to a 3 dB peak, but due to the limitations of the system, applied at a frequency slightly away from this peak, adjacent frequencies may be unnecessarily attenuated, and the desired “peak” not properly flattened.
The second problem can be at least partially offset using a brute force approach. If the equalization software was provided with an enormous number of narrow-band parametric equalization elements, then individual peaks and valleys could be selectively eliminated in the system response. However, such an approach may be processor-, memory-, and hardware-intensive.
The first problem can be resolved by using an equalizer that targets both magnitude and phase, if the system designer is not limited to the use of an existing magnitude-only equalization algorithm already present in a product.
U.S. Pat. No. 6,721,426 to Allred et al. discloses an automatic loudspeaker equalizer. First digital data is provided for a tolerance range for a target response curve of sound level versus frequency for the loudspeaker. Second digital data is generated for an actual response curve of sound level versus frequency for the loudspeaker. The first digital data is compared with the second digital data, and it is determined whether the actual response curve is within the tolerance range. If the actual response curve is not within the tolerance range, digital audio filters are iteratively generated, and the digital audio filters are applied to the second digital data to generate third digital data for a compensated response curve. The frequency, amplitude and bandwidth of the digital audio filters are automatically optimized until the compensated response curve is within the tolerance range or a predetermined limit on the number of digital audio filters has been reached, whichever occurs first.
The iterative approach of Allred improves equalization of the audio system, resulting in a flatter system response. However, the iterative approach can take considerable time to achieve. In particular, in the system of Allred, only one equalization element is adjusted with each iteration. As a result, it will take at least as many iterations as equalization elements to properly adjust all equalization elements and insure each equalization adjustment does not introduce new artifacts into the equalization. For a consumer electronic system (e.g., Home Theater system), this solution may not be acceptable, as the process may continue on for some time. The consumer may get impatient or believe the process if not functioning properly and terminate the process prior to completion.
Thus, it remains a requirement in the art to provide an equalization technique that more accurately equalizes a home theater or other audio system while using a limited number of equalization elements and/or working within the parameters of an existing equalization algorithm. It remains a further requirement in the art to provide an equalization technique that optimizes the use of equalization elements for a given audio environment. It remains a further requirement in the art to provide an equalization technique that can optimize equalization settings without requiring a large number of iterative time-consuming processes.